The issue is not in 2040, it's between now and 2040. For example, the current population in the 40-45 age range is nearly 2 million less than the 55-59 range. So sometime between now and 2040 (probably around 2030) we are going to see a decrease in the number of newly retired individuals. By 2040, the first wave of millennials will start retiring, which should make the 'silver tsunami' less of an issue.
I agree there will be a small peak, but there is no cliff on the other side. 5 years is a pretty small range to look at 10 years you get the following:
There are 37.5 million folks in their 60s now.
There will be 40 million folks in their 60s in 2030
there will be 39 million folks in their 60s in 2040.
Today, there are ~77 million Americans over 60. In 2040, there will be ~100 million Americans over 60.